The present invention relates to a method of atomizing liquids, preferably metal melts, in which liquid, preferably metal melt, is mixed into a media jet consisting of gas and/or liquid, so that the liquid is disintegrated into small particles, i.e. atomization is achieved. The invention also relates to a means for performing said method.
Such atomization is effected by disintegration of a preferably vertical tapping stream, or other pool of liquid, with the aid of preferably horizontal or vertical media flows consisting of gas or liquid.
When liquids are being atomized by disintegration of the liquid with the aid of a gas or fluid, extremely small particles are obtained within certain size intervals, the intervals sometimes being considerable. These known methods can be used for most types of liquids. However, they apply primarily to the production of powder from metal melts where a gas, e.g. nitrogen or argon, is used as atomization medium. Powder manufactured in this manner is often said to be manufactured inertly and is characterised by its low oxygen content and spherical form.
Powder-metallurgy processes using inertly manufactured powder encounter various problems relating to the size of the powder particles and/or their distribution. Finer and/or more restricted fractions of inertly manufactured powder are desirable for many applications nowadays. Such powder is conventionally obtained by screening off a coarser fraction, resulting in low yield, or via atomization processes using extreme gas flows and pressures. This powder is only used to a limited extent due to its high cost.
When atomizing metal melts in which a tapping stream is encountered by one or more gas jets, instability is produced on the surface of the melt in the contact surface between melt and gas, causing the melt to be stretched out in thin films. When these films have reached a certain thickness they will be broken up into threadlike pieces due to the surface tension of the melt and these pieces will be twisted off into a number of bits which assume a shape having the least possible surface energy, i.e. spherical shape.
These spherical drops solidify to powder particles extremely rapidly due to thermal radiation and convective dissipation of heat to the gas.
The size of particles formed in a certain volume element in the atomization process is affected by a number of parameters. The surface tension of the melt and the density and velocity of the atomizing medium are the most influencial parameters, besides the geometrical design of the atomization process.
It is difficult to influence the surface tension or density for a given melt, atomizing nozzle and atomizing medium, and it is therefore simplest to influence the particle size by means of the velocity of the atomizing medium. In most established atomizing processes, therefore, high velocities are strived for by means of high pressure in the atomizing medium and, in the case of gaseous media, by Laval design of the nozzles. However, the velocity of gaseous atomizing media decreases extremely rapidly after the nozzle so that usually only a small proportion of the atomizing process occurs within the region of maximum velocity.
A larger or smaller proportion of the melt will be disintegrated to particles in a region further away from the nozzle, where the velocity is considerably less, in some cases even as low at 10% of the maximum velocity. This gives a coarse powder with a wide spread between the smallest and largest particles.
Another problem entails the difficulty of getting the atomizing medium to get a "grip" on the liquid, and a large quantity thus passes outside the actual atomizing region, with low effectivity as a result.